Walking on ashes: insect trace fossils from Laetoli indicate poor grass cover associated with early hominin environments

More than 4000 insect trace fossils collected in recent years from Pliocene deposits at Laetoli in northern Tanzania provide new insights on early hominin palaeoenvironments. These trace fossils include: Fictovichnus gobiensis, Coprinisphaera murguiai, C. kheprii, Coprinisphaera ispp., Quirogaichnus isp., Teisseirei linguatus isp. nov., Celliforma ritchiei isp. nov., C. spirifer, C. germanica, C. cfr. curvata, Celliforma ispp., Rosellichnus isp., Vondrichnus planoglobus, Laetolichnus kwekai igen. et isp. nov. and Krausichnidae indet. They reveal that at least one species of moth, three dung beetles and five other coleopterans, nine taxa of solitary bees, and an indeterminate number of taxa of termites inhabited the Lower Laetolil environments. The Upper Laetolil environments, which have yielded a rich diversity of vertebrate fossils, including the early hominin Australopithecus afarensis and its putative footprints, supported several taxa of termites, one dung beetle, five other coleopterans, and eleven taxa of bees. The Upper Ndolanya environments, which have yielded the hominin Paranthropus aethiopicus, record four taxa of dung beetles, four other coleopterans, and two taxa of bees. The record of larval mortality and lack of intruder activity, revealed by the absence of emergence and intruder traces, may be associated with the anoxic/hypoxic conditions caused by the instantaneous burial of soils under thick volcanic ashes. The record of the Celliforma Ichnofacies in the Upper Laetolil environments indicates the dominance of shrubland to woodland with limited grass cover. This is supported also by the absence of Coprinisphaera, which suggests a scarcity of fresh grasses and a low abundance of large mammal grazers.

近年来，从坦桑尼亚北部莱托利（Laetoli）地区上新世（Pliocene）地层中采集的4000余件昆虫遗迹化石，为早期古人类古环境研究提供了全新视角。本次发现的遗迹化石包括：戈壁拟痕迹（Fictovichnus gobiensis）、穆尔吉亚粪球印迹（Coprinisphaera murguiai）、克普利粪球印迹（C. kheprii）、粪球印迹属未定种（Coprinisphaera ispp.）、基罗加迹未定种（Quirogaichnus isp.）、舌形泰斯迹新种（Teisseirei linguatus isp. nov.）、里奇细蜂迹新种（Celliforma ritchiei isp. nov.）、螺旋细蜂迹（C. spirifer）、日耳曼细蜂迹（C. germanica）、近曲细蜂迹（C. cfr. curvata）、细蜂迹属未定种（Celliforma ispp.）、罗塞利迹未定种（Rosellichnus isp.）、平球冯德迹（Vondrichnus planoglobus）、克瓦克莱托利迹新属新种（Laetolichnus kwekai igen. et isp. nov.）以及未确定克劳斯迹科（Krausichnidae indet.）。

这些遗迹化石表明，下莱托利尔地层环境中曾栖息至少1种蛾类、3种蜣螂、5种其他鞘翅目昆虫、9个独居蜂类分类单元，以及数量未定的白蚁类群。上莱托利尔地层环境产出了丰富的脊椎动物化石，包括早期古人类南方古猿阿法种（Australopithecus afarensis）及其疑似足迹，该环境曾栖息多个白蚁类群、1种蜣螂、5种其他鞘翅目昆虫以及11个蜂类分类单元。上恩多兰亚地层环境产出了古人类埃塞俄比亚傍人（Paranthropus aethiopicus）化石，该环境记录有4个蜣螂类群、4种其他鞘翅目昆虫以及2个蜂类分类单元。

遗迹中未发现羽化痕迹与侵入痕迹，这反映出幼虫死亡与无侵入活动的现象，其成因可能与厚层火山灰瞬时掩埋土壤所导致的缺氧/低氧环境相关。上莱托利尔地层环境中的细蜂迹遗迹相（Celliforma Ichnofacies）记录显示，该区域以灌丛与林地为主，草本植被覆盖有限。粪球印迹属（Coprinisphaera）的缺失也佐证了这一结论，这表明新鲜草本植物匮乏，大型植食性草食动物的丰度较低。